A MySQL function that throws an error in a `SELECT` statement will have its error suppressed in a `DO` statement.

do_or_not_do

DELIMITER |

CREATE FUNCTION checkit(doit INTEGER, message VARCHAR(256)) RETURNS INTEGER
BEGIN
    IF doit IS NULL OR doit = 0 THEN
        SIGNAL SQLSTATE 'ERR0R' SET MESSAGE_TEXT = message;
    END IF;
    RETURN doit;
END;
|

DELIMITER ;
select checkit(NULL, 'foo');
do checkit(NULL, 'foo');

-- Output:
-- Query OK, 0 rows affected (0.00 sec)
--
-- ERROR 1644 (ERR0R): foo
--
-- Query OK, 0 rows affected, 1 warning (0.00 sec)